Numerous technologies for forming flat-panel displays are known in the art. One such technology is the electro-luminescent display, which is formed by coating a thin layer of electro-luminescent material between a pair of electrodes. Displays employing this technology produce light as a function of the current between the two electrodes when the electro-luminescent materials are electrically stimulated. Electro-luminescent displays are primarily classified as active-matrix or passive-matrix displays. Active-matrix displays employ a relatively complex, active circuit at each pixel in the display to control the flow of current through the electro-luminescent material layer(s). The formation of this active circuit at each pixel can be expensive and often the performance of these circuits is somewhat limited. Passive-matrix displays are much simpler in their construction. Each pair of electrodes at each pixel is formed by the intersection of a row and a column electrode. As this type of display does not require the costly formation of active circuits at each pixel site, they are much less expensive to construct.
Referring to FIGS. 13 and 14, a prior-art display is illustrated having electrodes 12 and 16 with an electro-luminescent layer 14 formed between the electrodes 12 and 16 and responsive to a current provided by the electrodes 12 and 16 to produce light. The two electrodes 12 and 16 are typically patterned in orthogonal directions 8 and 6 over a substrate 10 and driven by external row and column drivers (not shown) connected to the electrodes 12 and 16.
While passive-matrix displays can be much less expensive to construct than active-matrix displays, they often suffer from relatively severe operational limitations, for example, resolution and refresh rate limitations, which restrict the commercial application of the passive-matrix displays to small, very low-resolution displays. Because of these limitations, the typical passive-matrix thin-film EL display is less than 2 inches in diagonal and has fewer than 150 lines of light-emitting elements. One of the more severe of these limitations occurs due to the fact that the thin-film EL display is formed from a very thin layer of relatively high-resistance EL material between a pair of metal electrodes. In this configuration, the EL pixel has a very high capacitance and when driving this pixel in a display, enough current must be provided to the pixel to overcome the capacitance before the pixel can emit light. Of course, the larger the pixel, and the thinner the electro-luminescent material, the larger the capacitance and the more energy that is required to overcome this capacitance before light is produced. Therefore, large displays employing thin films of electro-luminescent materials will require significant power to overcome the capacitance of the pixels in the display.
This power issue is further worsened for passive-matrix displays having a relatively higher resolution as these displays are typically addressed by placing a reference voltage on a single row electrode, e.g., second electrode 16 shown in FIGS. 13 and 14, in the display and then providing pixel voltages on each column line, e.g., first electrode 12, simultaneously. In this addressing scheme, a pre-charge current is provided to each pixel to overcome the capacitance of each pixel, current is provided to the EL pixels to produce light, the voltages are then changed to switch the row of pixels into reverse bias, draining the capacitance, and then the next line is addressed. To provide a flicker-free image, this process needs to be completed for each line in the display at a rate around 70 Hz. Therefore, as the number of lines on the display is increased, the amount of power that is dissipated by charging and discharging the capacitance of the light-emitting elements in the display increases. Further, it is necessary to turn on and off a large number of rows of data at the very high rates that occur when the display has a large number of lines (e.g., significantly more than 100 lines) that have to be refreshed at a rate of 70 Hz. Accordingly, it becomes very expensive to construct drivers that are capable of providing high enough currents to perform the required process of pre-charging each pixel, providing current to light each pixel, and then providing sufficient reverse bias in order to perform this refresh process. Therefore, it is not only necessary to reduce the amount of power that is dissipated in pre-charging and reverse biasing each light-emitting element, but to also reduce the peak current that must be provided by the drivers.
Many different solutions for overcoming or avoiding these problems have been suggested. For example, U.S. Pat. No. 6,980,182, issued Dec. 27, 2005 to Nimmer et al, entitled “Display System,” suggests patterning an insulating layer over a subset of the rows of the display before depositing the column lines, forming numerous layers of independently addressable row drivers. Different row and column drivers are then used to drive the different rows of the display within each layer of the row drivers. In this way, the amount of current that must be provided by any single driver is reduced as it is divided among two or more drivers. While this does make any single driver for the display less expensive, it requires multiple drivers, which can add significant cost to the overall system.
US Patent Application 2002/0101179, filed Dec. 27, 2001 by Kawashima, entitled “Organic Electroluminescence Driving Circuit, Passive Matrix Organic Electroluminescence Display Device, and Organic Electroluminescence Driving Method,” suggests driving the passive-matrix display using two power supplies. The first power supply serves as a “voltage holding” supply. The second of these power supplies is used to provide current to activate the light-emitting elements of the display (i.e., provide current to light each light-emitting element). In such a device, all but the active light-emitting elements are attached to the voltage holding supply. This power supply maintains the charge in the capacitors at or near the threshold of the light-emitting diodes such that the light-emitting elements do not have to be charged or discharged. Besides adding the cost of a second power supply, such displays will often have leakage current near this threshold, and therefore require power to be dissipated even when the display is intended to be dark, which of course also elevates the black level of the display somewhat as the light-emitting elements will produce a small amount of light in response to this leakage current.
A similar approach is employed in U.S. Pat. No. 6,486,607, issued Nov. 26, 2002, by Yeuan, entitled “Circuit and System for Driving Organic Thin-Film Elements,” which discusses an electronic circuit that allows the light-emitting elements to be pre-charged via the row line on the cathode while constant current is provided via the column line, attached to the anode. In this way, the light-emitting elements may be pre-charged by a power supply on the row drivers while a power supply on the column drivers is used to provide power to activate the light-emitting elements.
US Patent Application 2005/0219163, filed Apr. 25, 2002 by Smith et al., entitled “Display Driver Circuits for Organic Light-Emitting Diode Displays with Skipping of Blank Lines,” discusses constructing a driver that contains a frame buffer and image processing methods that makes it possible to analyze the information before it is displayed. In the approach that is discussed, each row of input data is analyzed to determine if any row is substantially black. If it is, the drivers skip the line while driving the display such that power is not wasted to pre-charge and then reverse bias each of the light-emitting elements within a row of pixels that will not be activated. Unfortunately, this approach will only reduce power under very specific display conditions and is not generally applicable to large graphic displays, which often employ text on white backgrounds; and, therefore, will rarely display a black line.
While each of the previously discussed approaches attempt to avoid the problems of power dissipation due to pre-charging and reverse biasing the light-emitting elements or reducing the current that any single driver is required to provide, each of these approaches apply the same basic drive technique. A different approach to driving a passive matrix display is employed in WO 2006/035248, filed Sep. 30, 2004 by Smith et al., however, which discusses an approach that allows all of the light-emitting elements of a display to be lit simultaneously. In such an approach, the driver employs a frame buffer to store an input image. This input image is then analyzed and a number of orthogonal pairs of matrices are formed and stored, which may be used to approximately describe the content of the image. One of the matrices in each orthogonal pair is then used to provide a signal to the row drivers while the second of the matrices in the same orthogonal pair is used to provide a signal to the column drivers. These row and column driver inputs are then updated to display each of the orthogonal pairs of matrices during each image update cycle. Using this method, pre-charging and reverse biasing of the light-emitting elements are avoided, reducing the overall power required to drive the passive matrix display and decreasing the instantaneous current load that is required from each of the drivers. Unfortunately, the image processing that is required to create the orthogonal pairs of matrices is significant, especially when such processing must be accomplished in real time and at rates of 30 Hz or higher. Further, the drivers must be equipped with significant memory and be capable of driving each row to several drive voltage levels. These features can add significant cost to the drive electronics, which are required to drive the thin-film EL display, significantly increasing the cost of the overall display system.
There is a need; therefore, for a method of controlling and driving passive-matrix displays that enables the use of lower-cost drivers, reduces the power consumption, and improves the resolution of the passive-matrix display.